With the age of highly computerized societies in recent years, communication machines and equipment for satellite broadcasting, mobile phones, etc., tend to be digitized, and signal processing tends to be quicker. These use printed wiring boards constituted of composite materials composed of materials such as a reinforcing material, a resin, a modifier, a filler, and the like. Further, glass fibers are widely used as a reinforcing material for their peripheral plastic members. Conventionally, E glass is known as a commercially produced glass fiber of this kind.
When an alternate current is flowed in glass, generally, the glass absorbs energy with regard to the alternate current and absorbs it as heat. The dielectric loss energy to be absorbed is in proportion to a dielectric constant and a dielectric tangent which are determined by the components and the structure of the glass, and is shown by the following expression. EQU W=kfv.sup.2.times..di-elect cons.tan .delta.
in which W is a dielectric loss energy, k is a constant, f is a frequency, v.sup.2 is a potential gradient, .di-elect cons. is a dielectric constant, and tan .delta. is a dielectric tangent.
The above expression shows that with an increase in the dielectric constant and the dielectric tangent, or with an increase in the frequency, the dielectric loss increases.
E glass, for example, has a dielectric constant of 6.7 and a dielectric tangent of 12.times.10.sup.-4 at a frequency of 1 MHz at room temperature, and a printed wiring board formed of E glass is insufficient for complying with demands of a higher density and a higher processing speed. There are therefore desired glasses which have a lower dielectric constant and a lower dielectric tangent than the E glass. Among them is a glass called D glass. D glass is for example, a glass having a composition containing 75.3% of SiO.sub.2, 20.5% of B.sub.2 O.sub.3, 0.6% of CaO, 0.4% of MgO, 0.6% of Li.sub.2 O, 1.1% of Na.sub.2 O and 1.5% of K.sub.2 O. For example, it has a dielectric constant of 4.3 and a dielectric tangent of 10.times.10.sup.-4 at a frequency of 1 MHz at room temperature.
However, D glass has the following defects. Since it has poor meltability and is liable to cause striae and foams, a glass fiber frequently breaks during its spinning step, and it is poor in productivity and workability. Further, since it has a very high spinning temperature, the lifetime of a furnace is decreased. Moreover, there is another problem that since D glass has poor water resistance, and since it has poor adhesion to a resin, it is liable to peel from a resin in a printed wiring board so that no high reliability can be obtained when it is used for a printed wiring board.
JP-A-6-219780 discloses a low-dielectric-constant glass comprising 50.0 to 65.0% of SiO.sub.2, 10.0 to 18.0% of Al.sub.2 O.sub.3, 11.0 to 25.0% of B.sub.2 O.sub.3, 6.0 to 14.0% of MgO, 1.0 to 10.0% of CaO and 0 to 10 of ZnO, MgO+CaO+ZnO being 10.5 to 15%. The above glass is intended to decrease a spinning temperature so as to improve productivity by particularly incorporating at least 6% of MgO and adjusting MgO+CaO+ZnO to at least 10.5%. Since, however, MgO, a component which is highly liable to undergo phase separation and give a high dielectric tangent, is incorporated in an amount of 6% or more, no sufficient water resistance can be obtained, and the dielectric tangent of the glass is relatively high.
JP-A-7-10598 discloses a glass having a composition containing 50.0 to 65.0% of SiO.sub.2, 10.0 to 18% of Al.sub.2 O.sub.3, 11.0 to 25.0% of B.sub.2 O.sub.3, 0 to 10.0% of CaO, 0 to 10.0% of MgO, 1.0 to 15.0% of the CaO+the MgO, 0 to 10.0% of ZnO, 0 to 10.0% of SrO and 1 to 10.0% of BaO. Since, however, the above glass contains, as an essential component, BaO which is a component to increase the dielectric constant, it is difficult to obtain a sufficiently low dielectric constant. For obtaining a low dielectric constant, it is inevitable to decrease the BaO amount, and in this case, there is a problem that the viscosity of the glass increases so that the spinning workability is poor. Further, there is another problem that since BaO corrodes a furnace material to a great extent, the lifetime of the furnace is decreased.
The present Applicant has already proposed, by Japanese Patent Application No. 7-137688, a glass comprising 50 to 60% of SiO.sub.2, 10 to 20% of Al.sub.2 O.sub.3, 20 to 30% of B.sub.2 O.sub.3, 0 to 5% of CaO, 0 to 4% of MgO, Li.sub.2 O+Na.sub.2 O+K.sub.2 O being 0 to 0.5%, and 0.5 to 5% of TiO.sub.2. The above glass retains a low dielectric constant and a low dielectric tangent and at the same time shows excellent properties during a spinning step with regard to workability and productivity. However, it still has some slight difficulty in workability and productivity.